A rectangular tank 6 × 5 × 4 ft holds about 3,398 litres (120 cubic feet). This free water tank capacity calculator converts any rectangular or cylindrical tank's dimensions in feet to litres, US gallons and UK gallons — and suggests a household tank size from your family's daily water use and how many days of backup storage you want.
How do you calculate water tank capacity in litres?
Find the tank's volume in cubic feet, then multiply by 28.32 — that is how many litres one cubic foot holds. For a rectangular tank: length × width × water depth. A tank 6 ft long, 5 ft wide and 4 ft deep is 6 × 5 × 4 = 120 cubic feet, or 120 × 28.32 ≈ 3,398 litres. For a cylindrical tank: π × (diameter ÷ 2)² × water depth. A 5 ft diameter tank with 6 ft of water is 3.1416 × 2.5² × 6 ≈ 117.8 cubic feet, or about 3,336 litres. Measure the inside water depth — the space below the overflow outlet — not the tank's outer height, because the freeboard at the top is never usable storage. In metric terms the rule is even simpler: 1 cubic metre holds exactly 1,000 litres.
How many litres of water does a family need per day?
A common planning figure is about 135 litres per person per day for a household with piped water — covering drinking, cooking, bathing, washing and cleaning. A family of six therefore uses roughly 800 litres a day. Actual use varies widely: careful households use far less, while homes with lawns, frequent car washing or leaking fittings use much more. Fixing dripping taps and running washing machines on full loads are the cheapest ways to cut consumption. Multiply daily use by the number of days you want to survive a supply interruption — two days is a sensible minimum where municipal supply is intermittent — to get your target storage.
What size water tank do I need for my house?
Match the tank to daily use × backup days. Typical guidance: a small family (3–4 people) with 2 days' backup needs around 800–1,100 litres; a family of 5–6 needs 1,300–1,700 litres; larger joint families often go to 2,000–3,000 litres. Plastic overhead tanks in Pakistan are commonly sold in 500, 750, 1,000, 1,500 and 2,000 litre sizes — the calculator suggests the nearest common size above your calculated need. Underground RCC tanks are built bigger (5,000 litres and up) and suit homes with very unreliable supply. Remember the roof's load capacity: 1,000 litres weighs a tonne, so large overhead tanks need proper structural support.
How do I use this calculator?
Choose the tank shape, then enter the inside dimensions in feet — length, width and water depth for rectangular tanks, or diameter and water depth for cylindrical ones. The capacity appears instantly in litres, US gallons and UK (imperial) gallons. Below that, the household sizer takes your family size, litres per person per day (135 is the default planning figure) and days of backup, and recommends the nearest standard tank size. Both parts update live as you type, so you can compare sizes quickly.
What are the limitations?
The calculator assumes a perfect rectangular or cylindrical shape filled to the water depth you enter — sloped bases, curved walls, internal partitions and irregular tanks need separate calculation. Household sizing is a rule of thumb: real consumption depends on habits, fixtures and season, and very hot months can push use higher. Tank nameplate sizes are nominal; usable capacity is always a little less because of freeboard. For underground RCC tanks and anything structural, have the design checked by a qualified engineer — water is heavy, and a failed tank is dangerous as well as expensive.
Frequently asked questions
How many litres are in 1 cubic foot?
One cubic foot holds 28.32 litres (28.3168 to be exact). One cubic metre holds 1,000 litres, and one cubic foot is 1/35.31 of a cubic metre.
What is the difference between US and UK gallons?
A US gallon is 3.785 litres; a UK (imperial) gallon is 4.546 litres. Pakistani tank capacities are almost always quoted in litres, so use litres when buying and treat gallons as a conversion for reference.
Why does my 1000-litre tank hold less than 1000 litres?
Tanks are rated by total internal volume, but the overflow outlet sits below the rim, leaving freeboard that cannot be used. Measure the water depth up to the overflow — not the outer height — for the true usable capacity.
Overhead or underground tank — which is better?
Overhead tanks give gravity-fed pressure and are cheap to install, but add a tonne or more of roof load per 1,000 litres. Underground tanks store more and stay cooler, but cost more to build and need a pump to lift water up.
How often should a water tank be cleaned?
At least once or twice a year. Sediment, algae and biofilm build up in stored water, especially in translucent tanks exposed to sunlight — opaque, food-grade tanks with tight lids stay cleaner longer.
Does hot weather change how much water we use?
Yes. Bathing, drinking and lawn watering all rise in summer, so size your tank for peak-season use rather than the annual average if supply is unreliable in hot months.
Updated October 2026 · Built and tested by Sharjeel Tahir · How our tools are tested